Switches are provided in communication networks in order to direct traffic to a desired destination. As the use of networks has increased over time, so has the need for more bandwidth. Fiber optic networks were developed to meet this need and transmit data (e.g., voice and data signals) at high data rates. The American-based Synchronous Optical Network (“SONET”) standard and the corresponding European equivalent standard, Synchronous Data Handling (“SDH”), are examples of two industry standards developed for the transmission of data over such fiber optic mediums. For simplicity the remaining description of optical-based networks will focus upon the SONET standard. However, those skilled in the art will recognize that the concepts as they pertain to SONET are also applicable to SDH and other data transmission protocols.
In a conventional SONET network, the network elements (or nodes), such as an add/drop multiplexers (“ADM”), have little or no information about other network elements in the network, thereby requiring manual intervention by a system administrator. In particular, conventional SONET networks required a system administrator to set up connection routes between ports coupled to the network elements of the network. A system administrator then would program the route in each network element along with the path from an ingress point to an egress point on the network. Typically, each network element in the network would have to be manually programmed to pass information in this manner. If a failure occurs in any one of the connections, the system administrator must manually reroute the connections by reprogramming the network elements.
In a SONET-based network data is transmitted as a series of multiplexed time slots or frames. The lowest data rate transmission typically within a SONET network is at a base rate of 51.84 Mbit/second, which in the electrical domain is referred to as a synchronous transport signal—level one (“STS-1”) frame, and in the optical domain corresponds to an Optical Carrier—level 1 (“OC-1”) frame.
Higher data rate frames can be formed from integer multiples of STS-1s, and are designated as STS-N/OC-N with N being a value, such as 3, 12, 48, 192, etc. For example, an OC-3 transmission is three times the base rate of OC-1.
As seen in FIG. 1, an OC-48 signal, when converted to corresponding electrical signals, includes 48 STS-1 frames. Each STS-1 frame is transmitted during a respective time slot, and comprises two components: a transport overhead and a payload. The transport overhead is provided in 9 rows of three bytes each (27 bytes total), and carries administrative information used by network elements to manage the transfer of the frame through the network. The payload, referred to as the Synchronous Payload Envelope (“SPE”), is provided in 9 rows of 87 bytes each (783 bytes total) and comprises the major portion of an STS-1. The SPE carries payload and STS Path Overhead (“STS POH”) bytes, and may begin at any byte location within the payload envelope, as indicated by a pointer value in the overhead.
Certain broadband transmission protocols (e.g., ATM), however, include relatively large payloads which do not fit within a single STS-1. Thus, in order for these protocols to be transmitted over SONET signal, a plurality of STS-1s are concatenated together. Such concatenated STS-1 are referred to as STS-Nc, and are multiplexed, switched and transported as a single unit. The SPE of an STS-Nc includes N×783 bytes, which may be considered as an N×87 column×9 row structure. Only one set of STS POH is used in the STS-Nc, with the pointer always appearing in the transport overhead of the first of the N STS-1s that make up the STS-Nc.
The SONET standard, however, requires that the STS-1s that make up an STS-Nc occupy specific time slots. For example, FIG. 2 illustrates 48 time slots occupied by 16 OC-3cs transmitted within an OC-48 frame. In particular, as seen in FIG. 2, OC-3c #1 must occupy a “row” of time slots 1, 17 and 33, OC-3c #2 must occupy time slots 2, 18, and 34, and OC-3c #16, must occupy time slots 16, 32, and 48. In order to add a new OC-3c, one entire row shown in FIG. 2 must be removed or reallocated.
FIG. 3 illustrates specific time slots occupied by four OC-12cs within an OC-48 frame. Specifically, OC-12c #1 must occupy time slots 1–4, 17–20, and 33–36, OC12-c #2 must occupy time slots 5–8, 21–24, and 37–40, and OC-12c #4 must occupy time slots 13–16, 29–32, and 45–48. Likewise, in order to add a new OC-12c, an entire row shown in FIG. 3 must be removed or reallocated.
If time slots 1, 2, and 3 are dropped in the OC-48 frame shown in FIG. 2, and populated with data for an OC-3c, however, they could not be switched by current SONET equipment because they are not transmitted in a sequence conforming to the concatenation protocol described above. Rather, a conventional SONET network element would need to be reconfigured to thereby rearrange the remaining time slots so that a new OC-48 frame is created which does conform to the standard concatenation sequence. If reconfiguration is not performed, the empty time slots cause bandwidth fragmentation.
Reconfiguring SONET network elements, however, requires substantial down time causing disruption in the flow of data through a network. Thus, there is a need for a network element, which can arbitrarily (flexibly) concatenate time slots associated with an OC frame which are not provided in a given “row” or sequence required by SONET.
SONET/SDH defines concatenated payload structures in (GRE-253 and ITU G.707) for STS-Nc where N is a multiple of 3. Only STS-3c, STS-12c, STS-48c, STS-192c concatenated structures are in wide use. The STS-Nc payload structures are required to occupy rigidly defined contiguous timeslots within the STS-N data stream. This rigid industry standard requirement results in timeslot fragmentation and inefficient bandwidth utilization in a network where traffic is mixed with STS-1 and STS-Nc connections.
Timeslot fragmentation occurs as connections are added and deleted. For example, if an STS-3c connection is added to an STS-48, three timeslots in the STS-N must be available and those three times slots must be contiguous. If this condition does not exist, other existing connections must be re-groomed to make room for the STS-3c. The re-grooming process results in a traffic hit for the existing connections. If the traffic is not re-groomed, then bandwidth fragmentation occurs.
Flexible concatenation doesn't rigidly require the STS-Nc to occupy contiguous timeslots. Rather, the only requirement is that the parent timeslot containing the pointer value arrives into the framer before the child timeslots containing the concatenation identifier. Flexible concatenation, therefore, does not have any issues with timeslot fragmentation. If an STS-3c connection is added, as long as three timeslots exists within the STS-N, it can be added without having to be re-groomed.
Such flexibly concatenated data is defined as any STS-Nc sub-rate connection that is not an STS-3c/12c signal (e.g. STS-24c) or is an STS-3c/12c sub-rate signal which contains time-slots not in the specific ordering specified by Telecordia (BellCore) GR-253, January 1999.
Inefficient bandwidth utilization occurs when higher layer traffic is groomed into an STS-Nc that is larger than required. For example, a Gigabit Ethernet data stream occupies less than 24 timeslots, but in the standard SONET/SDH concatenation structure must occupy the entire STS-48c. Flexible concatenation allows for a flexible size STS-Nc payload structure in an STS-N. The flexible size STS-Nc payload capability allows flexibility in the size of the concatenated payload. For example, the Gigabit Ethernet can be transported in an STS-24c, which results in better bandwidth utilization.
Potential problems may exist within a network when a non-transparent network element, which does not support flexible concatenation, (“non-standard network elements”) with respect to payload and pointer processing within a SONET/SDH network (or otherwise known as a facility) and such non-standard network elements are deployed between network elements that support flexible concatenation (“standard network elements”).
When a standard network element with an OC-48 interface is used to transport flexibly concatenated payloads between standard network elements, it is important that network elements between the standard network elements transparently pass the SONET/SDH Line Overhead (“LOH”). In particular, it is important that the intermediate facility equipment not perform any pointer justifications (the pointer bytes are in the LOH). If the intermediate facility equipment is not a CoreDirector™ intelligent optical switch, it will not be able to detect and determine the non-standard concatenated payload structure in the SONET/SDH signal to properly adjust the pointers. What ends up happening is that the intermediate facility equipment does not adjust each STS-1 payload consistently in the non-standard STS-Nc and this will result in a corrupted payload at the onset of the first pointer justification.
In flexible concatenation, the child timeslots of the STS-Nc contain the concatenation identifier in the pointer value and only the parent timeslots contain pointer values.
Network elements, such as CIENA Corporation's CoreDirector™ intelligent optical switch uses flexible concatenation on its line-side facilities. For example, when a Sub-Network Connection (“SNC”) utilizing flexible concatenation timeslots is established on a non-transparent facility, the SNC may not operate error free. This problem is due to the line-terminating network element in the middle of the network not forwarding the pointer bytes transparently across the network. These network elements instead perform pointer processing and regeneration only for standard concatenation timeslots and cannot perform pointer interpretation correctly for flexible concatenation.
Both the foregoing general description and the following detailed description explain examples of the invention and do not, by themselves, restrict the scope of the appended claims. The accompanying drawings, which constitute a part of this specification, illustrate apparatus and methods consistent with the invention and, together with the description, help explain the principles of the invention.